1. Field of the Invention
This invention relates to an alternating water/solvent/gas injection method for increasing the recovery of oil from a subterranean formation.
2. Background
Water alternating gas (WAG) processes have been used heretofore to recover hydrocarbons from subterranean formations. In such processes, water is first injected into the formation followed by a quantity of gas which is, in turn, followed with an additional slug of water with the steps being repeated a number of times. The sizes of the gas slugs typically used vary from 20% to 40% of the initial hydrocarbon pore volume of the formation. Gas compositions are usually maintained at a constant level and may be followed by either water or, in some instances, another gas. Such processes are generally used to improve the mobility control of the injected fluids in the formation. In other words, the water has a lower injected fluid/oil-mobility ratio than does the gas. Desirably, the ratio of rate of injected fluid flow through the formation to the rate of oil flow through the formation is less than 1. Values greater than 1 are undesirable since, in such instances, the injected material moves through the formation faster than the oil and tends to bypass oil in the formation, thereby channelling through the formation and bypassing many of the areas which it is desired to contact. Gas typically has an even higher mobility ratio which may be as high as 10 to 100. Obviously, gas will quickly channel through a formation bypassing many of the areas it is desired to contact.
Further, the use of pure component gases, such as carbon dioxide, tends to result in a multi-contact miscible environment in the formation. The term "multi-contact miscible environment" refers to an environment in which it takes multiple contacts for the gas and oil to mix, thereby eliminating the interfaces between the gas and oil to the point where the oil can be more easily recovered from the formation. This process generally takes multiple gas contacts with the oil and the like.
In a variation of this process, rich gas mixtures which consist of hydrocarbon gases containing from two carbon atoms to about five carbon atoms, are used. These rich gas mixtures are frequently liquid in the formation environment and function more as a solvent. The result is an environment referred to as a "first contact miscible environment". In such an environment, the solvent and oil mix on first contact and then the oil is more readily flowed from the formation. Such processes result in much better efficiency but they require the use of more expensive materials and as a result are used less frequently.
Multi-contact miscible environments using lean gases, i.e. pure component gases such as methane, nitrogen, carbon dioxide or mixtures thereof, which require multiple contacts or longer contact periods are used more frequently.
Since the recovery of additional oil from a subterranean formation is, in most instances, an expensive process, a continuing search has been directed to the development of more effective and efficient ways to recover additional oil from subterranean formations economically.